Process of Modification of a Feedstock in a Delayed Coking Unit

ABSTRACT

A process is described of delayed coking optimized for greater yield of diesel oil from coke by means of modifications to the feedstock in a Delayed Coking Unit. According to the present invention in a first embodiment of the invention the feedstock includes the bottom product of the vacuum distillation tower, known in the prior art as vacuum residuum and the heavy gas oil from coke obtained in the fractionation tower and recycled to comprise the aforesaid combined feedstock. In a second embodiment of the present invention the feedstock consists of: the bottom residuum proceeding from the atmospheric distillation tower, known in the prior art as atmospheric residuum, and heavy gas oil from coke obtained in the fractionation tower and recycled to comprise the aforesaid feedstock of the unit.

BACKGROUND OF THE INVENTION

The field of application of the present invention is delayed cokingprocesses. Particularly in delayed coking processes wherein the yield ofdiesel oil is maximised whereas the yield of heavy gas oil from coke isminimised through modifications to the feedstock of a Delayed CokingUnit.

DESCRIPTION OF PRIOR ART

The process of delayed coking of residual petroleum fractions has beenemployed in the petroleum refining industry for some time. This processpermits conversion of heavy petroleum fractions into lighter productshaving greater value added such as, for example, liquefied petroleum gas(LPG), naphtha, gas oils and coke.

In a conventional delayed coking process the new feedstock, generally avacuum residuum, is fed to the bottom region of the fractionation towerwherein incorporation of the natural recycle occurs forming the combinedfeedstock of the Unit. Normally the natural recycle is employed toadjust the quality of heavy gas oil from coke to be sent to any FluidCatalytic Cracking (FCC) Unit.

The combined feedstock is sent to a furnace wherein it must dwell for avery short time, of the order of a few minutes, such that the thermalcracking reactions can be initiated and the formation of coke in thefurnace tubes be minimised.

On leaving the furnace at a temperature of the order of 500° C. thecracked feedstock is fed to the coke drum wherein the thermal crackingand coking or carbonisation reactions are completed. These reactionsgenerate hydrocarbons lighter than those in the combined feedstock andcoke. The reactions which take place in a coke drum are endothermic andthe temperature of the effluents from the drum lie within a band ofvalues from 430° C. to 455° C.

The coke formed accumulates in the drum until it requires to be removedfollowing stages of steam purging and cooling with water. With theobjective of removing the accumulated coke in a coke drum the effluentfrom the coke drum is diverted to another empty coke drum wherein theaccumulation phase is initiated. Removal of the coke is carried out bymeans of high-pressure-water cutting devices.

The effluents from the coke drum are then sent to a fractionation towerof a Delayed Coking Unit wherein they are separated into:

-   -   A mixture of fuel gas, LPG and light naphtha exiting from the        top of the fractionation tower, known for this reason in the        prior art as top gases; and    -   Side drawings of heavy naphtha, light gas oil (LGO) from coke        and heavy gas oil (HGO) from coke.

In order to achieve better operational yield special care is taken atsome stages of the delayed coking process, i.e.:

-   -   It is desirable that coke formation occurs solely within a coke        drum and not within the tubes of the furnace. Thus the combined        feedstock dwells in the furnace for solely a few minutes in        order to minimise the formation of coke within the tubes        thereof; and    -   In order to prevent the reactions proceeding and possible        undesirable deposition of coke in the outlet tubing of the coke        drum a rapid cooling (quench) is carried out employing a stream        of gas oil and/or residuum.

With the discovery of increasingly-heavy petroleums the delayed cokingprocess in refineries has experienced an increase in its degree ofimportance, principally due to an increase in the yield of residuum fromsuch petroleums.

The delayed coking process is well-known in the prior art. One of theoldest processes is disclosed by U.S. Pat. No. 3,563,884. The aforesaidpatent describes a process wherein tar is utilised as raw material and aheavy gas oil recycle is provided for.

Some variations have been introduced based on the aforesaid invention.U.S. Pat. No. 4,213,846 discloses a delayed coking process for theformation of premium coke wherein the recycle is hydrotreated.

U.S. Pat. No. 4,177,133 describes a delayed coking process for theformation of premium coke wherein the new feedstock having passedthrough a preheating stage is subjected to flash distillation to removenon-crystalline substances.

U.S. Pat. No. 4,455,219 and U.S. Pat. No. 4,518,487 disclose a delayedcoking process wherein part or all of the heavy hydrocarbon productcommonly used as recycle is replaced by a lighter hydrocarbon, whichsame is combined with the new feedstock of the unit.

U.S. Pat. No. 4,661,241 describes a delayed coking process wherein theyield of coke is minimised and the yield of liquid products is maximisedby means of the elimination of recycle.

U.S. Pat. No. 5,711,870 discloses a process of delayed coking whereinthe fresh feedstock is mixed with water and, optionally, with a hydrogendonor such as methane or gas oil derived from the recycle in order tooptimise the yield of liquid products and reduce the yields of coke andgas.

As may be observed there is a tendency to develop delayed cokingprocesses with the objective of maximising the yield of liquid products,principally petrol, and reducing the yield of coke and gas. In order toachieve this objective there is a tendency to reduce the rate of recycleof the delayed coking process and increase the conditions of severity inthe vacuum distillation tower in order to maximise separation of heavyvacuum gas oil.

In this manner the quality of production of a heavy vacuum gas oilsuitable for use as feedstock for a Catalytic Cracking Unit isprioritised. This leads to the generation of increasingly heavy vacuumresiduums at the bottom of the vacuum distillation tower.

Thus for refining programmes wherein there are excesses of gas oil andvacuum residuum and greater demand for light gas oil from coke, thestate of the art is moving towards solutions making simultaneousconversion viable in order to maximise the yield of diesel oil from therefinery.

The diesel oil from a refinery comprises diverse streams, among themlight gas oil from coke produced in a Delayed Coking Unit. As thepresent invention described below refers to diesel oil produced fromlight gas oil from coke, hereinafter such diesel oil will be referred toas diesel oil from coke.

SUMMARY OF THE INVENTION

The process of modification of a feedstock in a Delayed Coking Unit,subject of the present invention, considers a solution maximising theyield of diesel oil from coke and minimising the yield of heavy gas oilfrom coke by means of modifications to the feedstock of a Delayed CokingUnit.

According to the present invention the feedstock consists of: the bottomproduct from the vacuum distillation tower, known in the prior art asvacuum residuum, and the heavy gas oil from coke obtained in thefractionation tower and recycled in order to comprise the aforesaidcombined feedstock. The percentage by volume of heavy gas oil from cokein the new feedstock lies within a band of values from 16% to 50%.Preferentially within a band of values comprised between 20% and 40%.

In a second embodiment of the present invention the feedstock consistsof: the bottom residuum proceeding from the atmospheric distillationtower, known in the prior art as atmospheric residuum, and the heavy gasoil from coke obtained from the fractionation tower and recycled tocomprise the aforesaid feedstock of the unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The process of modification of feedstock in a Delayed Coking Unit,subject of the present invention, will be better understood by means ofthe detailed description, given below solely as an example, inassociation with the drawings referred to below, which same are integralparts of this description.

FIG. 1 shows schematically a delayed coking process, according to theprior art.

FIG. 2 shows schematically a process of modification of a feedstock in aDelayed Coking Unit according to a first embodiment of the presentinvention.

FIG. 3 shows schematically a process of modification of a feedstock in aDelayed Coking Unit according to a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The description of the process of modification of a feedstock in aDelayed Coking Unit, subject of the present invention, will be providedin concordance with the identification of the respective components,based on the figures described above.

FIG. 1 shows schematically a delayed coking process according to theprior art. A fresh feedstock (1) is fed to a fractionation tower (2)whence diverse derivatives are removed such as, for example, fuel gasand LPG (3), light naphtha (4), heavy naphtha (5), light gas oil (6),medium gas oil (7) and heavy gas oil from coke (8).

The bottom product (9) of the fractionation tower (2) is fed to afurnace (10) such that the thermal cracking reactions may be initiated.The effluent from the furnace (11) is subsequently sent to a coke drum(12) wherein the thermal cracking and coking or carbonisation reactionsare completed, generating coke and an effluent from the coke drum (13)comprising light hydrocarbons. The effluent from the coke drum (13) issubsequently sent to the fractionation tower (2).

At the commencement heavy gas oil from coke (8) is sent to a FluidCatalytic Cracking Unit (not shown in the figure) wherein it is used asraw material for the production of petrol.

FIG. 2 shows schematically a process of modification of a feedstock in aDelayed Coking Unit according to the present invention. A freshfeedstock (1) is fed to a fractionation tower (2) whence severalderivatives are removed such as, for example, fuel gas and LPG (3),light naphtha (4), heavy naphtha (5), light gas oil (6), medium gas oil(7) and heavy gas oil from coke (8).

A fraction (8′) of heavy gas oil from coke (8) is added to the bottomproduct (9) from the fractionation tower (2). The percentage by volumeof the fraction (8′) of heavy gas oil from coke (8) in relation to thefresh feedstock (1) lies within a band of values from 16% to 50%.Preferentially the percentage by volume of the fraction (8′) of heavygas oil from coke (8) in relation to the fresh feedstock (1) lies withina band of values from 20% to 40%.

The aforesaid fraction (8′) of heavy gas oil from coke (8) may be addedto the bottom product (9) by means of a line external to thefractionation tower (2), according to the embodiment shown in FIG. 2.

Alternatively the aforesaid fraction (8′) of the heavy gas oil from coke(8) may be added to the bottom product (9) within aforesaidfractionation tower (2).

The feedstock thus combined (9′) is subsequently sent to the furnace(10) in order that the thermal cracking reactions may be initiated. Theeffluent from the furnace (11) is subsequently sent to a coke drum (12)wherein the thermal cracking and coking or carbonisation reactions arecompleted, generating coke and an effluent from the coke drum (13)comprising light hydrocarbons. The effluent from the coke drum (13) issubsequently sent to the fractionation tower (2).

FIG. 3 shows schematically a process of modification of a feedstock in aDelayed Coking Unit according to a second embodiment of the presentinvention. Petroleum (14) is fed to an atmospheric distillation tower(15) whence diverse derivatives are removed such as, for example, fuelgas (16), naphtha (17) and others not shown in this figure. In thismanner the feedstock of the Delayed Coking Unit is the bottom residuum(18) from the atmospheric distillation tower (15), known in the priorart as atmospheric residuum, and a fraction (8′) of heavy gas oil fromcoke (8) proceeding from the fractionation tower (2) is added to thebottom product (9) of the fractionation tower (2).

EXAMPLES

The present invention may be better understood by means of the examplesbelow. However the examples do not limit the present invention.

In the examples there have been employed an atmospheric residuum (AR)and a vacuum residuum (VR) having the properties according to Table 1:

TABLE 1 AR VR RCR (% w/w) 7.3 15.0 ° API 14.3 9.5 S (%) 0.67 0.74

Example 1

A vacuum residuum was processed in a pilot delayed coking unit withoutheavy gas oil from coke recycle. The temperature of the furnace was0.500° C. and the pressure at the top of the coke drum was 2 kgf/cm²g.Volume yields of 51.3% for diesel oil from coke and of 20.2% for heavygas oil from coke were obtained. The mass yield of coke was 24.5%.

Example 2

A vacuum residuum was processed in an industrial delayed coking unithaving a furnace temperature of 500° C. and pressure at the top of thecoke drum of 2 kgf/cm²g and a heavy gas oil from coke recycle rate of8%. Volume yields of 54.9% for diesel oil from coke and of 14.6% forheavy gas oil from coke were obtained. The mass yield of coke was 25%.

Example 3

A vacuum residuum was processed in a pilot delayed coking unit having afurnace temperature of 500° C., pressure at the top of the coke drum of2 kgf/cm²g and total recycle of heavy gas oil from coke. The volumeyield was 68.2% for diesel oil from coke. The mass yield of coke was26%.

Example 4

An atmospheric residuum was processed in a pilot delayed coking unit,without heavy gas oil from coke recycle, having a furnace temperature of500° C. and pressure at the top of the coke drum of 2 kgf/cm²g. Volumeyields of 53.5% for diesel oil from coke and of 27.7% for heavy gas oilfrom coke were obtained. The mass yield of coke was 13.5%.

Example 5

An atmospheric residuum was processed in an industrial delayed cokingunit having a furnace temperature of 500° C., pressure at the top of thecoke drum of 2 kgf/cm²g and a heavy gas oil from coke recycle rate of25%. Volume yields of 62.9% for diesel oil from coke and of 14.0% forheavy gas oil from coke were obtained. The mass yield of coke was 15.2%.

Example 6

An atmospheric residuum was processed in a pilot delayed coking unithaving a furnace temperature of 500° C., pressure at the top of the cokedrum of 2 kgf/cm²g and total recycle of heavy gas oil from coke. Thevolume yield was 72.6% for diesel oil from coke. The mass yield of cokewas 17%.

In the above examples there is noted an increase in yield in terms ofdiesel oil from coke and a reduction in yield of heavy gas oil from cokewith an increase in the recycle rate of the process. In this manner bymeans of the herein described present invention there occurs a growingincrease in the yield of diesel oil and a significant reduction in theyield of heavy gas oil from coke.

The description hereinbefore provided of the process of modification ofa feedstock in a Delayed Coking Unit, subject of the present invention,must be considered solely as a possible embodiment or embodiments andany particular characteristics introduced therein shall solely beunderstood to be something described to facilitate comprehension. Inthis manner they cannot be considered to limit in any way the presentinvention which is restricted to the scope of the claims below.

1. A process for modification of a feedstock in a delayed coking unitwherein: a fresh feedstock (1) is fed to a fractionation tower (2)wherefrom one or more derivatives are removed; a bottom product (9) ofthe fractionation tower (2) is fed to a furnace (10) such that one ormore thermal cracking reactions may be initiated; effluent from thefurnace (11) is subsequently sent to a coke drum (12) wherein thethermal cracking and coking and/or carbonisation reactions arecompleted, generating from the coke drum (13) coke and an effluentcomprising light hydrocarbons; the effluent from the coke drum (13) issubsequently sent to the fractionation tower (2); characterised in thatto the bottom product (9) of the fractionation tower (2) there is addeda fraction (8′) of heavy gas oil from coke (8).
 2. A process accordingto claim 1 in which the one or more derivatives comprise one or more offuel gas, LPG (3), light naphtha (4), heavy naphtha (5), light gas oil(6), medium gas oil (7) and heavy gas oil from coke (8).
 3. A processaccording to claim 1 or 2, characterised in that the percentage byvolume of the fraction (8′) of heavy gas oil from coke (8) in relationto the fresh feedstock (1) is from 16% to 50%, preferably from 20% to40%.
 4. A process according to claim 1, 2 or 3, characterised in thatthe fraction (8′) of heavy gas oil from coke (8) is added to the bottomproduct (9) by means of a line external to the fractionation tower (2).5. A process according to any preceding claim, characterised in that theaforesaid fraction (8′) of heavy gas oil from coke (8) is added to thebottom product (9) within the fractionation tower (2).
 6. A processaccording to any preceding claim, characterised in that the feedstock ofa delayed coking unit is the bottom residuum (18) of the atmosphericdistillation tower (15).